Printers are devices that print characters onto a printing medium such as paper or polyester film and are commonly controlled by a computer that supplies the image in the form of print commands. Some printers use a colorant-containing liquid which may be either a dye or a polymer. These liquids are termed "ink" in the printer industry. The printer forms images on the printing medium by delivering ink to the medium using a print head that creates the proper patterns of ink to record the image permanently.
One type of printer is the ink-jet printer which forms a small droplets of ink that are ejected toward the printing medium in a precise pattern of dots. When viewed at a distance, the collection of dots forms the image in much the same manner as photographic images are formed in newspapers. Ink-jet printers are fast, produce high quality printing, and are quiet because there is no mechanical impact during operation.
Typically, an ink-jet printer has a large number of individual ink nozzles that are arranged in arrays in the print head. The print head is supported in a carriage, and the ink nozzles are oriented in a facing, but spaced apart, relationship to the printing medium. The carriage and the printhead traverse over the surface or the medium again and again with the nozzles ejecting droplets of ink at appropriate times under the command of the computer. After each transverse by the print head, the printing medium is moved an increment in the direction lateral to the transverse and thereafter the carriage with the print head traverses the page again to deposit another swath. In this manner the entire pattern of dots that forms the image is progressively deposited one swath at a time by the print head.
In a thermal ink-jet printer the ejection of droplets is accomplished by heating a small volume of ink adjacent the nozzle, vaporizing a bubble of ink, and thereby driving a droplet of ink through the nozzle toward the printing medium. The droplets strike the medium and then dry to form "dots" that, when viewed together, form one swath of the permanently printed image.
In some types of printers the ink is stored in a reservoir that is mounted on the carriage along with the print head. Ink is then delivered by capillary action to the nozzles. In these printers the print head is a single-use, consumable, disposable unit that may be readily inserted and removed from the printer when the ink reservoir is exhausted. One such printer and the print cartridges for it are described in Hewlett-Packard Journal, February 1994, Volume 45, Number 1.
In the early stages of the development of thermal ink-jet printers, the useful life of a print head was usually determined by the length of time until the first nozzle failed. More recently the design of nozzles and print heads has so advanced that the life of the nozzles prior to failure has significantly lengthened. In other words, the supply of ink in a reservoir may now be exhausted before a nozzle failure is experienced. Thus, there now exists a need for a larger supply of ink to be available for print cartridges because of the extended nozzle life.
Simply increasing the size of the ink reservoir has not proved to be an acceptable solution however. Typically, a reservoir is supported on the printer carriage and moves with the print head. Increasing the size of the reservoir would necessarily increase the size and weight of the structure that supports and moves the carriage back and forth. This would cause the performance of the printer to suffer because of the increased mass of the carriage and would also significantly increase the cost of the printer.
Still another solution would seem to be to refill the empty print cartridges with replenishment ink. This would allow the print heads to be used again and again until nozzle failure. As of yet this approach has not proven to be reliable or satisfactory because of at least four significant problems.
The first problem and probably the most significant from the operator's point of view is how to transfer ink from a replenishment ink reservoir to the print cartridge while avoiding spillage and leakage. No operator wants to have his or her hands, clothing, or work areas stained by spilled ink.
The second problem is maintaining the operating pressure in the print cartridge during the next operating cycle. Normally, print cartridges operate at a pressure of approximately two inches of water below atmospheric pressure (3.74 torr), and the ink is supplied to the nozzles at this pressure by capillary action. In some print cartridges the pressure of the ink in the reservoir is maintained by a collapsible ink bag and a spring which urges the walls of the ink bag apart against atmospheric pressure. If the pressure of the ink exceeds a maximum level, ink will be forced out of the nozzles and the print cartridge will "drool" ink onto the paper and into the printer. If the pressure of the ink in the print cartridge drops below a minimum level, the flow of ink to the nozzles will stop because the capillary pressure is exceeded.
A third problem is maintaining the pressure of the ink in the print cartridge during refilling. If the pressure of the ink exceeds a maximum level during refilling, then ink will drool from the nozzles and leakage will occur. If the pressure in the print cartridge drops below a minimum level, then air may be drawn into the nozzles which may block the passage of ink and cause nozzle failure.
A fourth problem is the inadvertent introduction of air or gases into the print cartridge during replenishment. If bubbles are entrapped in the print cartridge during replenishment, these bubbles can travel within the print cartridge and block the narrow passage ways leading to the print nozzles and thereby cause nozzle failure.
It will be apparent from the foregoing that although there are many processes and apparatus for refilling print cartridges, there is still a need for an approach that avoids spillage and leakage and properly maintains the pressure within the print cartridge during refilling and the next operating cycle.